1. Field of the Invention
The present invention relates to a structure and a driving method of a semiconductor device having a transistor. In addition, the present invention relates to a structure and a driving method of an active matrix display device using a semiconductor device having a thin film transistor. Further, the present invention relates to an electronic device using such a semiconductor device in a display portion.
2. Description of the Related Art
In recent years, a so-called self-luminous display device having a pixel formed by using a light-emitting element such as a light-emitting diode has attracted attention. As a light-emitting element used for such a self-luminous display device, an organic light-emitting diode (also referred to as an OLED (organic light-emitting diode), an organic EL element, an electroluminescence (electro luminescence: EL) element, and the like) has attracted attention, and it has been used for an EL display or the like. Since a light-emitting element such as an OLED is a self-luminous type, an EL display or the like has advantages over a liquid crystal display such that it has higher pixel visibility, no backlight is needed, and response speed is higher, and the like. Luminance of a light-emitting element is controlled with the amount of current flowing thereto.
In addition, in recent years, an active matrix display device in which a light-emitting element and a transistor which controls light emission of the light-emitting element are provided in each pixel has been developed. An active matrix display device has been expected to be put into practical use because not only it can realize high-definition display and large-screen display which are difficult to realize in a passive matrix display device, but also it can operate with less power consumption than the passive matrix display device and has higher reliability.
When driving methods of pixels of an active matrix display device are classified according to kinds of input signals, a voltage programming method and a current programming method can be given as examples. The former voltage programming method is a method of controlling luminance of a light-emitting element with a driving element by inputting a video signal (voltage) which is to be input to a pixel to a gate electrode of the driving element. In addition, the latter current programming method is a method of controlling luminance of a light-emitting element by supplying programmed signal current to the light-emitting element.
Here, an example of a pixel structure of a display device to which a voltage programming method is applied, and a driving method thereof are described with reference to FIG. 67. Note that an EL display device is described as an example of a typical display device.
FIG. 67 is a diagram showing an example of a pixel structure of a display device to which a voltage programming method is applied (see Reference 1: Japanese Published Patent Application No. 2001-147659). The pixel shown in FIG. 67 includes a driving transistor 6701, a switching transistor 6702, a storage capacitor 6703, a signal line 6704, a scan line 6705, a first power supply line 6706, a second power supply line 6707, and a light-emitting element 6708.
Note that in this specification, description that a transistor is on corresponds to a state where gate-source voltage of a transistor is higher than the threshold voltage thereof and current flows between a source and a drain. Description that a transistor is off corresponds to a state where gate-source voltage of a transistor is lower than the threshold voltage thereof and no current flows between a source and a drain.
When the switching transistor 6702 is turned on in response to change in a potential of the scan line 6705, a video signal input to the signal line 6704 is input to a gate electrode of the driving transistor 6701. Gate-source voltage of the driving transistor 6701 is determined in accordance with a potential of the video signal which is input, and the amount of current flowing between a source and a drain of the driving transistor 6701 is determined. This current is supplied to the light-emitting element 6708, so that the light-emitting element 6708 emits light.
In this manner, the voltage programming method is a method of setting gate-source voltage of a driving transistor and the amount of current flowing between a source and a drain of the driving transistor in accordance with a potential of a video signal, and a light-emitting element emits light at luminance corresponding to this current.
As a semiconductor element which drives a light-emitting element, a polysilicon (p-Si) transistor is used. However, electrical characteristics such as the threshold voltage, on current, and mobility of a polysilicon transistor easily vary due to a defect in a crystal grain boundary. In the pixel shown in FIG. 67, when characteristics of the driving transistor 6701 vary between each pixel, the amount of drain current of the transistor 6701 varies in accordance with a video signal which is input even when the same video signal is input. Therefore, luminance of the light-emitting element 6708 varies.
In addition, in a conventional pixel circuit (FIG. 67), a storage capacitor is connected between a gate and a source of a driving transistor. In the case of forming this storage capacitor using a MOS transistor, a channel region of the MOS transistor is not induced when gate-source voltage of the MOS transistor is approximately equal to the threshold voltage of the MOS transistor. Therefore, the MOS transistor cannot function as a storage capacitor. Accordingly, video signals cannot be accurately held.